Gap junctions consist of ensembles of intercellular channels that mediate electrotonic, metabolic, and dye coupling among cells of many tissues. Conductance through gap junction channels of fish and amphibian embryos is gated by transjunctional voltage, cytoplasmic pH, and to a lesser extent, by Ca ions. By the combined use of electrophysiological and optical techniques we intend to determine whether these gating mechanisms are common to other cell types, localize gating regions of the channel macromolecule, correlate junctional conductance with permeability and determine biophysical properties of individual gap junction channels. Although no disease state is now attributable to alterations in gap junctions, role in pathological conditions have been postulated (e.g., cataract formation, tumorigenesis, cardiac arrhythmias). Detailed analysis of permeation of gap junction channels may indicate the plausibility of a role in pathology and also provide information generalizable to other smaller ion channels.